1. Field of the Invention
The present invention relates to an optical recording and reproducing apparatus, an optical head, an optical recording and reproducing method and a skew detection method suitable for use with an optical recording medium which can be recorded and/or reproduced with irradiation of near-field light.
2. Description of the Related Art
Optical recording mediums (or magneto-optical recording mediums) represented by a CD (Compact Disc), a MD (Mini Disc) and a DVD (Digital Versatile Disc) are widely used as storage mediums to store music information, video information, data, programs and the like. In the systems for recording and reproducing these optical recording mediums, it has been customary that an objective lens opposes the recording surface of the optical recording medium in a non-contact fashion to read very small recording marks by detecting very small concavities and convexities formed on the recording surface of the optical recording medium or a reflectance change structure of a phase-change material. If a recording medium is a magneto-optical recording medium, it has been customary to read very small recording marks by detecting a magnetic domain structure in which a Kerr rotation angle changes.
In recent years, since it is requested that such optical recording medium should be larger in storage capacity and higher in recording density, technologies to read smaller recording marks from the optical recording medium with high resolution are now under examination.
The size of a beam spot of light irradiated on the optical recording medium is substantially given by λ/NA where λ is the wavelength of light irradiated on the optical recording medium and NA is the numerical aperture of a focusing lens to focus this light on the optical recording medium. Resolution also is in proportion to the value of λ/NA. The numerical aperture NA is expressed as:NA=n×sin θIn the above equation, n is the refractive index of the medium and θ is the angle at which marginal ray is introduced into the optical recording medium by an objective lens. The NA is never greater than 1 insofar as the medium is the air and hence resolution has a limit. For this reason, in the optical recording and reproducing apparatus, its light source has been improved. For example, a wavelength of a semiconductor laser has been shortened and a numerical aperture of a focusing lens has been increased.
On the other hand, a so-called near-field optical recording and reproducing system using evanescent waves, that is, light attenuating exponentially from the interface is proposed as a method that can achieve a numerical aperture larger than 1. In this near-field optical recording and reproducing system, it is necessary to decrease a gap between the focusing lens and the surface of the optical recording medium considerably.
An optical recording and reproducing method using a solid immersion lens (hereinafter simply referred to as a “SIL”) is proposed as a method of recording and reproducing an optical recording medium with irradiation of near-field light to the optical recording medium (see Cited Patent Reference 1 and Cited Non-Patent Reference 1, for example).
As an optical recording and reproducing recording medium which can be recorded and/or reproduced by irradiating near-field light from a near-field light irradiating unit such as the SIL thereto, there is proposed a phase-change type recording type optical recording medium shown in FIG. 1, for example.
FIG. 1 of the accompanying drawings is a schematic cross-sectional view showing an arrangement of an example of such phase-change type optical recording medium. An optical recording medium, generally depicted by reference numeral 10 in FIG. 1, includes a substrate 1 made of a suitable material such as glass or polycarbonate (PC) on which a reflection film 2 made of a suitable material such as aluminum (Al), a dielectric layer 3 made of a suitable material such as SiO2, a phase-change material layer 4 made of a suitable material such as GeSbTe and a dielectric layer 5 made of a suitable material such as SiO2 are laminated in that order. Alternatively, there is proposed a read-only optical recording medium shown in FIG. 2. FIG. 2 is a schematic cross-sectional view showing an arrangement of an example of such a read-only optical recording medium. This read-only optical recording medium, generally depicted by reference numeral 10 in FIG. 2, includes a substrate 1 made of a suitable material such as glass and polycarbonate (PC) on which a reflection layer 2 made of a suitable material such as aluminum (Al) with pits corresponding to recording information are formed is formed (see Cited Non-Patent References 2 and 3, for example) FIGS. 1 and 2 show the states in which a near-field light irradiating unit 26 such as the SIL is opposed to the surface of the optical recording medium 10 with a very small gap therebetween to irradiate incident light Li to the surface of the optical recording medium 10.
According to the report (see Cited Non-Patent Reference 4, for example), when the near-field light irradiating unit such as the SIL is in use, it is desirable that a space (gap) between the surface of the near-field light irradiating unit and the surface of the optical recording medium, that is, a gap should be made less than 1/10 of the wavelength of light irradiated onto the surface of the optical recording medium.
For this reason, when the wavelength of light is short, it is unavoidable that the surface of the optical recording medium and the surface of the near-field light irradiating unit are located with an extremely small gap therebetween. There is a large possibility that the near-field light irradiating unit such as the SIL will hit the surface of the optical recording medium. If the near-field light irradiating unit hits the surface of the optical recording medium, there is then the risk that the optical recording medium will be damaged at its portion in which information is recorded. In order to prevent or avoid the above-mentioned disadvantage, there is proposed an arrangement of an optical recording medium shown in FIG. 3 (see Cited Non-Patent Reference 5, for example).
FIG. 3 is a schematic cross-sectional view showing an arrangement of such optical recording medium. As shown in FIG. 3, a protective layer 8 having a thickness larger than approximately 1 μm is formed on the uppermost surface of the information recording surface of the optical recording medium 10. In FIG. 3, elements and parts identical to those of FIG. 1 are denoted by identical reference numerals and therefore need not be described.
In this case, as shown in FIG. 3, focus position of light irradiated by the near-field light irradiating unit 26 is set to the surface of the recording and reproducing layer, in the illustrated example, the surface of the phase-change material layer 4 through the protective layer 8. Also, it is necessary that the gap between the surface of the protective layer 8 of the optical recording medium 10 and the surface of the near-field light irradiating unit 26 made of a suitable device such as the SIL should be made, also in this case, less than approximately 1/10 of the wavelength of irradiated light.
On the other hand, there is proposed an arrangement in which a skew margin between the SIL and the surface of the optical recording medium can be kept by forming the tip end of the SIL like a cone (see Cited Non-Patent Reference 6, for example).
FIG. 4 is a schematic cross-sectional view showing an arrangement of an example of the above-mentioned arrangement. As shown in FIG. 4, the tip end portion of the SIL is shaped like a cone except a flat surface with a diameter D of approximately 40 μm, for example. According to the cone-like tip end portion of the SIL, it is possible to keep a skew margin between the surface of the SIL and the surface of the optical recording medium 10.
[Cited Patent Reference 1]: Japanese Published Patent Application No. 5-189796
[Cited Non-Patent Reference 1]: I. Ichimura et al., “Near-Field Phase-Change Optical Recording of 1.36 Numerical Aperture”, Japanese Journal of Applied Physics, Vol. 39, pp. 962-967 (2000)
[Cited Non-Patent Reference 2]: M. Shinoda et al., “High Density Near-Field Optical Disc Recording”, Digest of ISOM2004, We-E-03
[Cited Non-Patent Reference 3]: M. Furuki et al., “Progress in Electron Beam Mastering of 100 Gb/inch 2 Density Disc”, Japanese Journal of Applied Physics, Vol. 43, pp. 5044-5046 (2004)
[Cited Non-Patent Reference 4]: K. Saito et al., “A Simulation of Magneto-Optical Signals in Near-Field Recording”, Japanese Journal of Applied Physics, Vol. 38, pp. 6743-6749 (1999)
[Cited Non-Patent Reference 5]: C. A. Verschuren et al., “Towards cover-layer incident read-out of a dual-layer disc with a NA=1.5 solid immersion lens”, Digest of ISOM2004, We-E-05
[Cited Non-Patent Reference 6]: M. Shinoda, “High-Density Near-Field Readout over 50GB Capacity Using Solid Immersion Lens with High Refractive Index”, Japanese Journal of Applied Physics, Vol. 42, pp. 1101-1104 (2003)